The present invention relates, in general, to an asynchronous motor for a driving mechanism, especially for a hoist or running gear, and more particularly to an asynchronous motor with controllable rotational speed and direction of rotation.
The publication, entitled "Betriebsanleitung Einbau-lmpulsgeber EG" (Operating Manual for Built-in Pulse Transmitter EG) by Mannesmann Demag Fordertechnik AG, describes a sliding rotor asynchronous motor with a mechanical brake for controlled drive mechanisms, in particular hoists or running gears. The described motor is arranged in a motor housing, with both end faces being closed off by an end shield that forms a cover. The asynchronous motor includes a rotary disk arranged on the rearward side of the motor shaft opposite the output side. The rotary disk is provided with individual magnets arranged on the marginal region of the disk at a constant angular separation from each other and at the same radial distance from the motor shaft. Hall sensors facing the magnets are arranged at a same level as the magnets. During each cycle the rotary disk rotates, the Hall sensors are passed successively by the individual modulation elements so as to produce at the output of the Hall sensors an alternating signal that is a function of the rotational angle. The rotational speed of the electric motor can be determined from the alternating signal, which makes it possible to control or regulate the rotational speed of the asynchronous motor. A second Hall sensor is provided to detect the rotational direction, with the second Hall sensor being arranged--as viewed in the rotational direction--behind the first Hall sensors at a same level as the first Hall sensors. The phase of the two Hall sensors is shifted with respect to one another, and the phase shift is used to determine the rotational direction.
This asynchronous motor suffers the drawback that the signal transmitter cannot be used with a motor having a cylinder rotor and a magnetic brake, since in this case magnetic fields cause a significant interference, and only limited space exists between the short-circuit winding and the end shield.
Other known types of asynchronous motors with a signal transmitter for determining the rotational speed and rotational direction include an encoder disposed on the motor shaft outside the motor. This type of asynchronous motor, however, is more expensive and is large in size.